The invention relates to hydraulic lift mechanism and in particular to such mechanism as is required to serve intermittent alternating vertical displacement of a load, wherein the load may be of various magnitudes within the capacity of the mechanism. Such conditions exist for hydraulically operated cranes and hoists, and the present invention is particularly concerned with clamshell devices as the means of picking up and discharging a loading of a crane or hoist.
Clamshell load-handling devices are of various configurations but are of a nature to require two cable suspensions--(a) for carrying the load, i.e., the clamshell device plus such load as it may contain and (b) for determining shell-opening and/or closing action of the device. The two cables are run from separate winches, with associated brakes and clutches, and relatively great operator skill is required in the selective sequential, concurrent and directional control of the winches and their cables, in order to perform the lift, drop and dig functions expected of the clamshell device.
Conventional cranes and hoists employ a prime mover such as a diesel engine or one of various types of electric motor, depending upon the design capacity of the involved lift system, and the rated power of the prime mover is conservatively selected for assured handling of the maximum rated load of the system. In most cases, the system further requires a gearbox, a speed reducer, a pulling drum (or winch) and a safety brake. Illustratively, for example, a crane with a 1-ton lift capacity (at one meter/second) requires a prime mover of 15 horsepower, and a crane with 10-ton lift capacity (at the same one meter/second) requires a prime mover of 150 horsepower. These requirements are not reduced when a clamshell device must be an operational component of the crane or hoist system.